Electrochromic materials change color upon application of a voltage. Electrochromic devices are commonly used in windows, rear view automobile mirrors, and flat panel displays.
The change in color of an electrochromic material is usually due to an oxidation/reduction ("redox") process within the electrochromic material. Most electrochromic devices are responsive in the visible light region. Electrochromic materials active in the visible spectral region include metal oxides, such as WO.sub.3, MoO.sub.3 and nickel oxides. Metal oxides typically range in color from highly colored, such as dark blue, to transparent.
Conducting polymers are a new class of electrochromic materials which have recently received attention. Oxidation or reduction of a conducting polymer, which changes its color and conductivity, is usually accompanied by an inflow or outflow of counterions in the conducting polymer known as "dopants". Common dopant counterions include ClO.sub.4.sup.- and BF.sub.4. For example, the conducting polymer poly(pyrrole) is dark blue and conductive in its oxidized state ("doped state"). In its reduced state ("de-doped state"), poly(pyrrole) is pale green and non-conductive. Similarly, poly(aniline) is nearly transparent in its reduced state. When oxidized with dopants, such as Cl.sup.- and SO.sub.4.sup.2-, poly(aniline) becomes dark green. During doping of a conducting polymer, the conducting polymer swells due to the absorption of solvated counterions and solvent.
Few electrochromic materials are capable of modulating infrared light, i.e., altering the wavelength and intensity of light in the infrared region. In fact, most electrochromic materials capable of any change in the infrared region are static electrochromics, i.e., materials which cannot be switched between different reflective states. Conducting polymers are known to exhibit electrochromism in the visible and infrared regions. Current electrochromic devices which modulate light in the infrared region are radiators rather than modulators.
There are two types of electrochromic devices--transmissive mode devices and reflective mode devices. In transmissive mode electrochromic devices, light passes through the device. The incident light is modulated as it passes through the device. In contrast, reflective mode electrochromic devices reflect incident light. The incident light traverses the device before being reflected. As the incident light and reflected light traverses the device, the light is modulated.
An important characteristic of an electrochromic device is its "dynamic range". The dynamic range of an electrochromic device is the difference in percent reflectance between the extreme electrochromic states of the electrochromic material at a given wavelength. Other important properties of an electrochromic device are multicolor capability, broad band response, switching time and cyclability. Cyclability is the number of times the color of an electrochromic device may be changed before significant degradation of the working electrode occurs.
There is a need for flexible flat panel displays of variable area for use as camouflage for military vehicles and personnel. In particular, the displays need to be capable of multi-spectral and tailorable operation in the visible through near infrared to long wave infrared regions, approximately 0.35 to 24 .mu.m. There is a special interest in devices capable of operating in the long wave infrared region beyond 8 .mu.m. The displays need to be thin and able to draped over objects of varying shapes and sizes.
An example of an electrochromic device which incorporates a conducting polymer is U.S. Pat. No. 5,253,100, issued to Yang ("the Yang patent"). The Yang patent discloses an electrochromic device containing two layers of poly(aniline) as the electrochromic material. The first layer is preferably poly(aniline)/poly(styrene sulfonate) (PSS.sup.-) or poly(aniline)/acrylate obtained by electrochemical polymerization of poly(aniline) in poly(styrene sulfonate) (PSS) or poly(acrylic acid). The second (less dense) layer is prepared by chemical polymerization of aniline monomer in a template of acrylic acid or HPSS to yield a processible composite which can be coated. The device further contains solid electrolyte. The solid electrolyte includes typical solid electrolyte components such as poly(vinyl sulfonate), poly(styrene sulfonate), poly(acrylic acid, salt) (PAA), poly(-2-acrylamido-2-methyl propane sulfonic acid) and poly(phosphazenes).
The device of the Yang patent has a sandwich structure which preferably comprises the following layers in the order recited: glass, indium tin oxide (ITO), solid electrolyte, second layer of poly(aniline), first layer of poly(aniline), ITO, and glass. The solid electrolyte is in contact with the second layer of poly(aniline). The second poly(aniline) layer is said to aid solid electrolyte penetration and contact with the first poly(aniline) layer. Yang discloses that the combination of the solid electrolyte and the two layers of poly(aniline) increases the efficiency of the device.
The Yang device is inflexible due to the glass outer layers. The glass outer layer can not be substituted with poly(ethylene), since ITO and all other transparent conductors cannot be deposited on poly(ethylene) without cracking. If the first ITO layer were replaced by a thin metal layer, such as gold, there would be a trade off of thickness versus opacity. The gold layer at a thickness required for efficient device operation would be substantially opaque. Furthermore, the layers of glass, ITO, and electrolyte are opaque to infrared light; the electrochromic material of the Yang device is not responsive in the infrared region.
Maricle et al., U.S. Pat. No. 3,844,636, discloses a reflective mode electrochromic mirror utilizing WO.sub.3 as the electrochromic material. The electrochromic material is sandwiched between a glass front layer and an ion porous layer. The ion porous layer is composed of a conductive reflective material. An electrolyte layer is adjacent the ion porous layer. A counter electrode layer is adjacent the electrolyte layer, such that the electrolyte layer separates the ion porous layer from the counter electrode layer. The electrochromic material, WO.sub.3, is infrared opaque and incapable of infrared modulation. The mirror is inflexible due to the glass front layer. Flexibility cannot be achieved by substituting plastics for the glass, since WO.sub.3 is incohesive, cracks, and peels on flexible substrates.
Castellion, U.S. Pat. No. 3,807,832, discloses a reflective mode electrochromic mirror. The electrochromic material may be a metal oxide such as WO.sub.3 or MoO.sub.3. The electrochromic material is sandwiched between a transparent conductive electrode and a layer of electrolyte. A porous reflective layer is disposed in the electrolyte. A counter electrode is in contact with the electrolyte. The electrochromic materials, metal oxides such as WO.sub.3 and MoO.sub.3, are opaque in the infrared region and incapable of modulation of light in the infrared region. The mirror is inflexible since the electrochromic material and the transparent conductive electrode are composed of materials which are inflexible.
Bennett et al., U.S. Pat. No. 5,446,577, ("the Bennett et al. patent") discloses a reflective mode electrochromic display comprising the following layers in the order recited: first transparent layer, electrolyte, electrochromic material, metallized electrode, and second transparent layer. Conducting polymers, including poly(aniline), poly(pyrrole), poly(thiophene), poly(phenylene sulfide), and poly(acetylene), may be employed as the electrochromic material. However, poly(phenylene sulfide) and poly(acetylene) are electrochromically poor or inactive. The conducting polymer is paired with a dopant, such as a sulfate or chloride. In the examples in the Bennett et al. patent, the electrolyte is an aqueous acidic solution containing poly(acrylic acid). Liquid propylene carbonate and solid poly(ethylene oxide) are disclosed as alternative electrolytes.
The Bennett et al. device is unable to modulate light in the infrared region for several reasons. First, since the electrolyte layer is contained between the transparent outer layer and the electrochromic material, an incident beam of light must traverse the electrolyte before interacting with the electrochromic material. All of the electrolytes disclosed by Bennett et al. absorb a substantial amount of light in the infrared region. Therefore, the electrolyte layer seriously impedes infrared signals reflected by the Bennett et al. device. Second, all the combinations of conducting polymers and dopants described modulate light in the infrared region extremely poorly or not at all. Lastly, with the exception of poly(ethylene), all the transparent layers described are substantially infrared opaque.